1. Field of the Invention
The present invention relates in general to multijunction photovoltaic devices and their method of manufacture. More particularly, the present invention relates to amorphous silicon multijunction photovoltaic devices constructed to have improved efficiency and stability.
2. Description of the Related Art
Photovoltaic (PV) devices are used to convert radiation, such as solar, incandescent, or fluorescent radiation, into electrical energy. This conversion is achieved as a result of what is known as the photovoltaic effect. When radiation strikes a photovoltaic device and is absorbed by an active region of the device, pairs of electrons and holes are generated. The electrons and holes are separated by an electric field built into the device.
In accordance with a known construction of solar cells using amorphous silicon, the built-in electric field is generated in a structure consisting of p-type, intrinsic (i-type), and n-type layers (p-i-n) of hydrogenated amorphous silicon (a-Si:H). In photovoltaic cells having this construction, the electron-hole pairs are produced in the intrinsic layer of the cell when radiation of the appropriate wavelength is absorbed. The separation of the electrons and holes occurs under the influence of the built-in electric field, with the electrons flowing toward the region of n-type conductivity and the holes flowing toward the region of p-type conductivity. This flow of electrons and holes creates the photovoltage and photocurrent of the photovoltaic cell.
It is important that the semiconductor material used for making a photovoltaic device is capable of absorbing and converting to useful electrical energy as much of the incident radiation as possible in order to generate a high yield of electrons and holes. In this regard, amorphous silicon is a desirable material for use in photovoltaic devices since it is capable of absorbing a high percentage of the incident radiation relative to other materials used in the construction of photovoltaic cells, such as polycrystalline silicon. In fact, amorphous silicon is capable of absorbing about 40% more of incident radiation than polycrystalline silicon.
In a p-i-n type amorphous silicon photovoltaic cell as presently known in the art, the undoped (intrinsic or i-type) layer between the p-type layer and the n-type layer is proportionally much thicker than the p-type and n-type layers. The intrinsic layer serves to prevent the radiation-generated electrons and holes from recombining before they can be separated by the built-in electric field. This structure is generally referred to as "p-i-n" if the radiation is incident on a p-type layer, and "n-i-p" if the radiation is incident on an n-type layer.
Some of the incident light is also absorbed by the doped layers (the p-layers and the n-layers). Because the carriers generated in these layers have an extremely short carrier lifetime, they recombine before they can be collected. Hence, absorption in the doped layers does not contribute to the photocurrent of the photovoltaic cell and a minimization of absorption in doped layers enhances the short-circuit current of p-i-n photovoltaic cells. Absorption loss in the p-layer is a function of the bandgap of the p-layer. Thus, by adjusting the bandgap of the p-layer, the absorption loss in the p-layer can be minimized by including in the p-layer a bandgap widening material such as carbon, nitrogen, oxygen or fluorine. For example, the p-layer can be provided as hydrogenated amorphous silicon carbide (a-SiC:H) with p-type doping.
However, the addition of bandgap widening material to the p-layer increases its resistance. Therefore the amount of bandgap widening material that is added is usually limited by the amount of resistance considered tolerable in the device.
The n-type layer functions to form a rectifying junction with the intrinsic layer. In order to enhance this function, it is desirable to provide the n-layer with a high conductivity. However, it is also desirable to provide the n-layer with a wide optical bandgap since, as described above, carriers generated therein do not contribute to the photocurrent of the cell. Unfortunately, as is the case of the p-layer, the addition to the n-layer of any of the bandgap widening elements described above results in an increase in the resistance of the n-layer. Therefore, the n-layer is typically provided with a concentration of a bandgap widening element that is limited by the amount of resistance considered tolerable in the device.
It is desirable to increase the total number of photons of differing energy and wavelength which are absorbed in order to maximize the photocurrent output of a photovoltaic device. One technique for increasing photon absorption, and thereby increase device efficiency, is to provide a multijunction photovoltaic device with two or more photovoltaic cells arranged in a stacked configuration, i.e., one on top of the other. Such multijunction photovoltaic devices, also known in the art as a tandem junction solar cell, are disclosed in U.S. Pat. No. 4,272,641 issued to Hanak (the '641 patent) and U.S. Pat. No. 4,891,074 issued to Ovshinsky and Adler, which are incorporated herein by reference. In particular, these patents teach the construction of tandem junction amorphous silicon solar cells, wherein each cell has the above described p-i-n structure.
Such multijunction photovoltaic devices consist of a stack of two or more photovoltaic cells which are both electrically and optically in series. Typically in such devices, short wavelength light is absorbed in a first, topmost cell, and longer wavelength light is absorbed in second and, if present, subsequent cells. The first, second and subsequent photovoltaic cells of the multijunction device preferably respectively have successively narrower optical bandgaps in order to efficiently absorb solar radiation.
In order for such multijunction p-i-n photovoltaic devices to operate at maximum efficiency, current must flow unimpeded from each photovoltaic cell to the next adjacent cell in the stack of cells. However, the nature of the multijunction p-i-n photovoltaic device, i.e., p-i-n-p-i-n . . . , results in an n-p junction occurring at each interface between adjacent p-i-n cells and therefore in series electrically with those adjacent cells. Disadvantageously, each of these n-p junctions represents a diode having a polarity opposite to that of the photovoltage generated by each of the adjacent photovoltaic cells. The n-p junctions are non-linear elements that oppose the flow of photocurrent and thereby impose a substantial power loss on the device.
FIG. 1 illustrates a plot of current vs. voltage (IV) of a multijunction p-i-n photovoltaic device. In particular, curve 100 (broken line) represents the IV characteristic for such a photovoltaic device in which no steps have been taken to overcome the adverse effect of the n-p junctions at the interfaces between adjacent cells. As illustrated by curve 100, an inflection occurs in the region where the photocurrent of the device changes direction. Such an inflection represents an undesirable increase in the series resistance of the device due to the n-p junction. This aspect of the IV curve, characteristic of the n-p junction, limits the amount of photocurrent that can be conducted by the photovoltaic device, and therefore lowers the fill factor and power generation capability of the device. As used herein, the fill factor of a photovoltaic device is the ratio V.sub.mp I.sub.mp /I.sub.L V.sub.OC, where V.sub.mp and I.sub.mp are respectively the voltage and current at maximum power delivery of the device, and V.sub.OC and I.sub.L are respectively the maximum voltage and current achievable in the device.
A solution to the above described problem caused by the n-p junctions is to modify the structure of the multijunction device so that the junction occurring between each pair of adjacent cells performs like a tunnel junction (i.e., a recombination junction). One known method for creating a tunnel junction between adjacent solar cells of a multijunction photovoltaic device constructed from crystalline semiconductor materials, such as silicon, is to heavily dope the respective n- and p-layers of the n-p junction formed by the adjacent cells. However, this method for creating a tunnel junction cannot readily be applied to the above described multijunction p-i-n devices because amorphous silicon is not easily doped to yield a highly conducting film. Such difficulty in achieving suitably high conductivity is particularly the case with wide bandgap alloys such as hydrogenated amorphous silicon carbide (a-SiC:H) and hydrogenated amorphous silicon nitride (a-SiN:H) which are preferred materials for constructing the p- and n-type layers of amorphous silicon p-i-n photovoltaic devices since, as described above, their use tends to maximize the optical transmissivity of each photovoltaic cell of the multijunction device. As a result, an attempt to highly dope the p- and n-layers of an amorphous silicon multijunction p-i-n device constructed with wide bandgap alloys does not achieve a desirable tunnel junction characteristic at the n-p junction between adjacent cells.
A method for creating a tunnel junction between adjacent solar cells of an amorphous silicon multijunction p-i-n device is disclosed in the above-incorporated U.S. Pat. No. 4,272,641. There, an additional tunnel junction layer is disposed between adjacent p-i-n cells, such layer being provided as a cermet incorporating a metal, or as a thin metal layer and a cermet, hereinafter the "metallic layer." While the metallic layer may function in conjunction with the adjacent cell layers to reduce the above described inflection in the IV curve 100 of the device (FIG. 1), the provision of the extra metallic layer substantially inhibits the manufacturing process. In order to manufacture an amorphous silicon multijunction photovoltaic device using such a metallic layer, a first photovoltaic cell is formed in a first material deposition system, for example a glow discharge chamber as described in the '641 patent. Next, the device must be removed from the glow discharge chamber and placed in a second material deposition system where the metallic layer is deposited. For example, the '641 patent describes deposition of the metallic layer by a sputtering process. Then the device must be returned to the first deposition system where a second photovoltaic cell is formed on the metallic layer. Of course, if the device includes more than two cells, the process of transferring between the two deposition systems must be continued. The additional deposition system and the time required to manufacture a multijunction device in accordance with such a process results in an overall increased cost of the device and reduction in production yield.
Further, while the metallic layer disclosed in the '641 patent is optically transmissive, it has a lower index of refraction than that of the adjacent n- and p-type a-Si:H layers of the cells it is disposed between. As a result of the different indexes of refraction, light is undesirably reflected at the interfaces between the metallic layer and the adjacent a-Si:H layers.
As stated above, the most efficient photovoltaic device will absorb all the light which impinges on it and convert the energy from the light into current. However, the total current produced by a multijunction device is equal to the smallest amount of current generated by one of its photovoltaic cells. Therefore, the design of multijunction devices is constricted in the sense that additional photovoltaic cells cannot be arbitrarily added to the device to ensure that all the impinging light is absorbed and converted into current. Because the first photovoltaic cell of a multijunction device absorbs a large portion of the impinging light, the second and subsequent photovoltaic cells typically are constructed with extra thick intrinsic layers in order to maximize the light absorbed and the current produced by the cell. Further, the intrinsic layers of the second and subsequent cells are typically made thicker to compensate for the absorption of light in the p- and n-type layers. Therefore, it has been generally recognized that the overall initial efficiency of such a multijunction device increases with the increase in thickness of the intrinsic layers of the second and subsequent cells.
A problem results, however, in that the photodegradability of a photovoltaic cell increases with increasing thickness of the intrinsic layer used in the photovoltaic cell. Therefore, there is a trade-off between initial efficiency and long term stability of a photovoltaic device.